24 CHANGES IN MILK CAUSED BY BACTERIA. 



Under favorable conditions of food, temperature, environ- 

 ment, etc., the possibilities of bacterial multiplication are enor- 

 mous. The temperatures to which milk is exposed in the 

 summer, when not refrigerated, are highly favorable to their 

 multiplication. 



Numbers of bacteria in market milk. Hill and Slack (19) 

 have tabulated results of examination of 2,394 samples of milk 

 in Boston, covering a period of four months (June to Sep- 

 tember), with results as follows : 



TABLE I. 

 BACTERIA IN BOSTON MILK. Per cent. 



Below 100,000 bacteria per cc. ... 42. 



Between 100,000 and 500,000 per cc. ..'. ... 29.75 



Between 500,000 and 1,000,000 per cc.. 9.75 



Between 1,000,000 and 5,000,000 per cc. ... .. 12.75 



Above 5,000,000 per cc... 5. 

 Uncountable plates ' 75 



These figures are not exceptionally high, but are good as 

 compared with others. Bacterial counts running into the 

 millions are common in hot weather. Rosenau (34) found 

 that the milk of Washington, D. C., averaged 22,134,000 bac- 

 teria per cc. in 1906 and 11,270,000 per cc. in 1907 during the 

 months of July, August and September. This 'writer gives a 

 resum of results of similar work done elsewhere. 



Effect of temperature on bacterial growth. At 1C. (34F.) 

 bacterial growth is very powerfully restrained for awhile. Milk, 

 after six or eight days at this temperature, may show less bac- 

 teria than when freshly drawn. The circumstance is attribut- 

 able to the "germicidal " phenomenon of milk. Pennington 

 (32) in the study of samples of milk kept for long periods at 

 1C. found that certain bacteria multiply at that temperature. 

 In time the numbers may run into the millions per cc. The 

 species belonged to the group of organisms capable of acting on 

 protein, without affecting the appearance of the milk. These 

 observations were made upon the best milk obtainable, as well 

 as upon poor milk. It must be concluded that cold storage 

 cannot be relied upon to keep milk indefinitely. At 10C. 

 (50F.) milk may be kept for two or three days without an 



